The invention relates to a hydraulic cylinder, especially as a pit prop or moving cylinder unit in underground mining, with a locking ring arranged on an outer cylinder tube sealing it at its end with a lead through for an axially movable rod element accommodated in it, whereby the locking ring is provided in the lead through surrounding the rod element with a sealing arrangement resting on the outer side of the rod element.
Hydraulic cylinders of this construction find frequent application in underground mining as telescopic pit props for fact support. They find application in other areas or mining also, for instance as moving cylinders in self-advancing supports, where they serve to move a conveyor in the direction of the face advance and then by retraction to retrieve a support frame connected to them.
As a consequence of the rough operating conditions underground, after a relatively short operational period the hydraulic cylinders used there incur damage to the sliding surfaces between the outer cylinder tube and the inner rod element, which often lead to loss of sealing. The region of the locking ring is here especially problematical at which an escape of hydraulic fluid first arises on damage to the rod element on its outside and/or damage to the sealing arrangement.
A hydraulic cylinder constructed as a hydraulic double telescopic prop of the generic construction is known from DE 196 47 943 C1. In this known hydraulic cylinder guide rings in the two locking rings between the outer tube and the intermediate tube on the one hand and the intermediate tube and the inner rod on the other hand cater for the guidance of the extensible parts with very little radial play, and the additionally provided rod sealing rings accepted in grooves in the locking ring can undertake their sealing function against an escape of hydraulic fluid in an especially reliable manner.
The known arrangement has proved itself in practice; it is however a disadvantage that, because of the small play between the guide rings and the rod elements guided by them (intermediate tube and piston rod) these are subject to a high degree of wear on their outer circumference and the sealing has therefore even here a limited life time.
It is the aim of the present invention to produce a hydraulic cylinder of the construction quoted in the opening paragraph, with an especially long life sealing arrangement, which at the same time realises the highly accurate Guidance during extension and retraction of the telescopic elements with the smallest possible play in the radial direction.
Accordingly the present invention is directed to a hydraulic cylinder as described in the opening paragraph of the present specification, in which the sealing arrangement has a guide ring provided with grease grooves open to the outer side of the rod element, which can be supplied with lubricant for the grease grooves via a lubricant line accessible on the outside on the hydraulic cylinder and which is arranged in an accepting groove formed in the locking ring between an axially inner and an axially outer collar, whereby the gap formed between the outer collar and the rod element is greater than the gap between the rod element (and the inner collar and that the guide ring is arranged axially inside adjacent to an axial further outward lying O-ring seal.
The construction according to the invention has the especial advantage that a film of lubricant is built up on the outer side of the rod element through the grease grooves provided in the guide ring, which effectively prevents any direct contact between the rod element and the guide ring guiding it and thereby facilitates an almost friction-free extension and retraction of the rod element. Since the lubricant feeder is accessible from outside on the hydraulic cylinder, as necessary either manually or with the aid of an automatic greasing device, lubricant can be regularly replenished, so that xe2x80x9cdry runningxe2x80x9d of the rod element in the cylinder tube does not occur.
The inner collar thereby forms on its end surface facing away from the guide ring the greatest possible abutment for the rod sealing ring. The larger gap formed between the outer collar and the rod ensures that superfluous lubricant does not escape in the direction of the inner collar and therewith in the direction of the rod sealing ring, but in the opposite direction where the lubricant is presented with only a smaller resistance, so that it can easily be forced outward in the direction of the wiping ring and can be forced under this and thereby any pollutants which have possibly entered can be carried out again.
Advantageously the guide ring is arranged in the sealing arrangement between an outer wiper ring and a rod sealing ring arranged axially further inward on the locking ring. The outer wiper ring ensures that on retraction of the rod element any dirt adhering to its outside is wiped off and is not transported into the inside of the hydraulic cylinder. The guide ring with its lubricant reliably distributed over the entire surface of the rod element via the guide grooves, preferably an environmentally friendly grease, reliably prevents the remainder of the dirt which has not been removed by the wiper ring, or moisture, being able to penetrate as far as the rod sealing ring, so that this is kept always absolutely clean and thus can maintain its sealing function over a long period.
Especially the sealing arrangement has a second guide ring at an axial separation from the first guide ring. Expediently the rod sealing ring is arranged between the two guide rings.
In a preferred embodiment the first and/or the second guide ring comprises a hard woven material. Advantageously they can be singly or multiple divided, which significantly eases their assembly in the locking ring.
Especially the first guide ring is divided once and has a somewhat axial or diagonally running division, into which the lubricant line opens. The distribution of the lubricant is effected here in an especially simple manner in that the lubricant first reaches the joint and from this into the grease grooves.
In a preferred embodiment the lubricant line has a first section running radially in the locking ring with an opening in the region of the first guide ring and a second section, which runs axially through the locking ring and at whose front end can be filled via a non-return valve. In this form of embodiment it is ensured that not only is the sealing arrangement completely accepted in the locking ring, but also the associated lubricant supply which then can be exchanged as a unit with the sealing arrangement by exchanging the locking ring, when this becomes necessary after a long operating period of the hydraulic cylinder.
Advantageously the O-ring seal is arranged between the first guide ring and the wiper ring. Owing to the excellent lubrication in addition to the good sealing provided it is possible to provide the rod element with a plastics material coating, which owing to the use of the new construction of guide ring with grease grooves is no longer damaged by the guide ring as in the state of the technology and which thus forms a long-lasting protection of the rod element against corrosion.
Especially when used as a pit prop the hydraulic cylinder according to the invention can be made multi-stage, with an inner prop element, an inner tube element surrounding it and an outer cylinder again surrounding the latter. The inner tube element thereby forms on one side the cylinder tube for the rod element and represents on the other side a, rod element itself for the outer cylinder tube as is well known in multi-stage hydraulic cylinders.